RED 5.6 should be fine with this version of OSX; the only real connection I can think of might be the conjunction of the hardware of a 2009 MacBook Pro and 10.11.6, although I don't think this should be too likely either.
If it's indeed crashing, can you relay me a copy of the Crash Report (preferably in a Text or Word Processor format) to firstname.lastname@example.org? Also, are you using RED as a stand-alone program or are you using it as a filter in an NLE? Or, does this problem happen in both situations?
When using RED 5.6 inside my host (Adobe Premiere CC 2017), it is flawless. I use it both as a filter, and I often use the RED transition plug-in.
RED is only unstable when I use it as a stand-alone, outside the host. Thanks for having me check up on that. And it appears to crash when I change the workspace from Standard to Library Browser.
This leads me to another question: does this version of RED NOT allow moving individual windows to a second monitor? When I try to customize the workspace, I am confined to one monitor./
Thanks for your help.